1. Field of the Invention
The present invention relates to the material of an insulating film necessary for fabricating a thin film transistor (TFT), and a method of manufacturing the insulating film material. More particularly, the present invention is well suited for application to an electro-optical device which is typified by a liquid crystal display panel or an electro-luminescence (EL) display device of active matrix type wherein pixel units and driver circuits are disposed on an identical substrate, and to an electronic equipment in which such an electro-optical device is installed. Incidentally, here in this specification, an expression “semiconductor device” is intended to signify general devices which can function by utilizing semiconductor properties, and it shall cover within its category, an electro-optical device which is typified by a liquid crystal display device of active matrix type fabricated using thin film transistors, and an electronic equipment in which such an electro-optical device is installed as a component.
2. Prior Art
There has been developed a thin-film transistor (hereafter referred to as a TFT), having an active layer made from a crystalline semiconductor film, which is crystallized by a method such as laser annealing or thermal annealing from an amorphous semiconductor film, formed on an insulating substrate having light transparency characteristics, such as a glass. The substrate mainly used in the manufacture of the TFT is a glass substrate such as a barium-borosilicate glass or alumino-borosilicate glass. This type of glass substrate has inferior resistance to heat when compared with a quartz substrate, but has the advantages of a low market price, and the fact that a large surface area substrate can be easily manufactured.
The structure of the TFT can be roughly divided into a top gate type and a bottom gate type, with respect to the arrangement of a gate electrode. In the top gate type, an active layer is formed on an insulating substrate such as a glass substrate, and a gate insulating film and a gate electrode are formed in order on the active layer. Furthermore, there are many cases in which a base film is formed between the substrate and the active layer. On the other hand, a gate electrode is formed on a similar substrate in the bottom gate type, and a gate insulating film and an active layer are formed in order on the gate electrode. In addition, a protective insulating film or an interlayer insulating film is formed on the active layer.
The gate insulating film of the TFT is manufactured from a film such as a silicon oxide film, a silicon nitride film, or a silicon oxynitride film. The reason that these types of materials are used is because in order to form a good interface with respect to an amorphous silicon film or a crystalline silicon film forming the active layer, it is preferable to form the insulating films from a material having silicon as one of the principal constituents.
It is considered as preferable to manufacture the above insulating films by plasma CVD or low pressure CVD. Plasma CVD is a technique of decomposing a raw material gas in a glow discharge, forming a radical (meaning here a chemically activated one) by being made into a plasma, and depositing this on the substrate. In plasma CVD, it is possible to deposit a film at a low temperature of normally 400° C. or less. However, ions also exist within the plasma, and therefore it is necessary to skillfully control the damage to the substrate due to ions accelerated by an electric field occurring in a sheath region. On the other hand, low pressure CVD is a method of thermally decomposing a raw material gas and depositing a film on the substrate. There is no damage to the substrate due to ions, as with plasma CVD, but low pressure CVD has the disadvantage of slow deposition speed, so it cannot always be applied to manufacturing process in view of circumstances.
It is required of a gate insulating film to sufficiently lower an interface state density and a defect level density (bulk defect density) in the film. It is also required to consider an internal stress and the magnitude of change thereof attributed to a heat treatment. It is important for forming the gate insulating film of good quality to prevent the introduction of interfaces and defects into the film in the course of the deposition of the film, and to prepare a composition adapted to lower the defect level density of the formed film. Expedients each of which employs a starting gas exhibiting a high efficiency of decomposition, have been thought out for that purpose. By way of example, a silicon oxide film which is manufactured by plasma CVD with a mixed gas consisting of TEOS (Tetraethyl Ortho Silicate whose chemical formula is Si(OC2H5)4) and oxygen (O2) is the insulating film of good quality. It has been known that, when a MOS structure is fabricated using the silicon oxide film and is subjected to a BT (Bias Temperature) test, the fluctuation of a flat-band voltage (hereinbelow, expressed by “Vfb”) can be diminished to a practicable degree.
Since, however, water (H2O) is liable to be produced and is easily entered into the film in the course of decomposing the TEOS by glow discharge, thermal annealing needs to be performed at 400° C.-600° C. after the formation of the film in order to obtain the good quality film as stated above. Unfavorably it becomes a factor for the increase of a fabrication cost to incorporate such a high-temperature annealing step into the fabricating process of a TFT.
On the other hand, a silicon nitride film which is manufactured from, for example, SiH4, NH3 and N2 by plasma CVD can offer a dense and hard film. Since, however, the silicon nitride film has a high defect level density and a high internal stress, it gives rise to a distortion at its interface defined with an active layer. Accordingly, it exerts the bad influences of shifting a threshold voltage (hereinbelow, expressed by “Vth”) and enlarging a sub-threshold constant (hereinbelow, shortly termed “S value”), on the characteristics of a TFT.
Further, a silicon oxynitride film which is manufactured by plasma CVD with a mixed gas consisting of SiH4 and N2O can offer a film of high density in such a way that several-several tens atomic % of nitrogen is contained in the film. Under some manufactural conditions, however, defect levels due to Si—N bonds appear, and the value of a voltage Vfb fluctuates greatly in a BT test. Even when the film is stable in the BT test, it lacks in a thermal stability, and the voltage Vfb is caused to fluctuate by a heat treatment at 300° C.-550° C. Such fluctuations in the characteristics can be conjectured ascribable to the change of the composition of the silicon oxynitride film.
Meanwhile, there has been known a technique wherein a silicon oxynitride film is manufactured by plasma CVD with a mixed gas consisting of SiH4, N2O and H2. By way of example, a thesis “Structural and optical properties of amorphous silicon oxynitride”, Jiun-Lin Yeh and Si-Chen Lee, Journal of Applied Physics, vol. 79, No. 2, pp. 656-663, 1996, discloses hydrogenated silicon oxynitride films which were manufactured by plasma CVD in which a decomposing temperature was set at 250° C., the mixing ratio of hydrogen (H2) to SiH4+N2O was held constant at 0.9 to 1.0, and the mixing ratio of SiH4 and N2O as expressed by Xg═[N2O]/([SiH4]+[N2O]) was changed from 0.05 to 0.975 inclusive. With Fourier-transform infrared spectrometry (FT-IR), however, it was clearly observed that HSi—O3 bonds and H2Si—O2 bonds existed in the hydrogenated silicon oxynitride films manufactured here. Such bonds are conjectured, not only to exhibit inferior thermal stabilities, but also to form defect levels around the bonds due to the fluctuations of coordination numbers. In such a case, despite the silicon oxynitride film, unless the composition thereof or the constituents thereof including impurity elements is/are examined in detail, the film cannot be easily used for a gate insulating film which exerts serious influences on the characteristics of a TFT.